Thin-film BAW filters are used, for example, in third-generation (3G) mobile radio technology in order to achieve a wide bandwidth with a low insertion loss at the same time.
Filters such as these are formed, for example, by CRF components (CRF=Coupled Resonator Filter). In general, a CRF component has two piezoelectric resonators, which are stacked one above the other, and a number of coupling intermediate layers in order to set the acoustic coupling to the desired filter characteristics.
CRF technology allows the required bandpass characteristics (that is to say the bandwidth and the insertion loss) to be achieved for all mobile radio bands. The cut-off areas, which are well away from the pass areas, have a very high insertion loss, which is governed to a significant extent by parasitic effects and by the finite conductivity of the substrate, as well as any asymmetric capacitances with respect to the substrate. However, the cut-off band values close to the pass area—the so-called transitional areas—are not optimum. In general, the mobile radio transmission and reception bands are separated from one another by transitional bands with a width of only 10 to 20 MHz, in which the filter characteristics including an insertion loss of typically better than 4 dB must change to better than 30 dB cut-off bands.
This disadvantage in the transitional bands between −5 dB and −30 dB is a fundamental characteristic of the CRF component itself and can also scarcely be improved by component design without accepting deterioration in the pass band characteristics.
One possible approach in order to obtain transitional bands with better characteristics would be to connect a CRF component to a conventional ladder filter device in the form of a cascade, as is illustrated by way of example in FIG. 8. FIG. 3 illustrates the transfer function of a ladder filter component, which has two pronounced minima associated with the series resonance of the parallel resonators and the parallel resonance of the series resonators in the ladder filter component. The illustrated example is a single T section. The layer stack which is used here is the same as that in the arrangement shown in FIG. 1, but is modified by removal of the upper electrode of the lower resonator and by its replacement by silicon dioxide. The width of the transitional bands is approximately 10 MHz for the left-hand transitional band, and 5 MHz for the right-hand transitional band, between −5 dB and −30 dB. Q factors of 1000 were used for the resonators.
FIG. 4 shows a typical layer stack of a conventional resonator which is used for ladder filters. If a single ladder filter is coupled in a cascaded form to the unbalanced port of the CRF, this results in a transfer function as shown in FIG. 5. In this case, the port can be operated as an input or output. A similar effect is achieved by using two ladder filters at the balanced outputs of the CRF. This combined CRF ladder filter has much better transitional band characteristics than a CRF on its own. Definitions of these terms can be found in the following publications and in the literature references cited in them:    G. G. Fattinger, R. Aigner, W. Nessler, “Coupled Bulk Acoustic Wave Resonator Filter: Key Technology for single-to-balanced RF Filters.” Proceedings IEEE 2004 MTTS Symposium Digest.    G. G. Fattinger, J. Kaitila, W. Nessler, and R. Aigner, “Single-to-Balanced Filters for Mobile Phones using Coupled Resonator BAW Technology”, IEEE UFFC Symposium 2004 Proceedings,    K. M. Lakin, “A Review of Thin-Film Technology”, IEEE Microwave Magazine, December 2003, p. 61.    K. M. Lakin, “Thin Film Resonator Technology”, IEEE UFFC 50th Anniversary Issue.    K. M. Lakin, “Coupled Resonator Filters”, IEEE 2002 Ultrasonics Symposium Proceedings.